1. Field of the Invention
The present invention relates to a motor control system which corrects backlash.
2. Description of the Related Art
The feed axis in machine tools and industrial machinery and the axis of arms etc. of industrial robots (mechanical moving parts) have servo motors coupled with them. Rotation of the servo motors is converted by ball screws etc. to linear motion of tables etc. The speed of transmission of the servo motors is slowed by reduction gears.
There is sometimes a difference between the stopping position in a positive direction to a certain position and the stopping position in a negative direction in these ball screws or reduction gears. In general, such a difference is called “backlash” and is a cause of reduction of positional accuracy.
FIG. 10A and FIG. 10C are views for explaining backlash. FIG. 10A shows a moving part WA which moves by a not shown motor and a driven part WB which is driven by the moving part WA. The moving part WA has projecting parts A1 and A2 at its two ends, while the driven part WB has a projecting part B at its center. Therefore, for example, if the moving part WA moves in the right direction, the inside end of the projecting part A1 of the moving part WA engages with one end of the projecting part B of the driven part WB. Due to this, the moving part WA and the driven part WB move integrally in the right direction.
Further, when the moving part WA reverses and moves from the right direction to the left direction, as shown in FIG. 10B, the moving part WA moves in the left direction. Further, as shown in FIG. 10C, when the inside end of the other projecting part A2 of the moving part WA engages with the other end of the projecting part B of the driven part WB, the moving part WA and the driven part WB move integrally in the left direction.
Before the moving part WA engages with the driven part WB at the time of reversal in this way, it is necessary for it to move by exactly a predetermined amount of movement called “backlash”. FIG. 10A and FIG. 10C show the backlash C. The backlash C can become a cause of reduction of the positional accuracy.
For this reason, the practice has been to prepare a correction amount for the backlash C and add this correction amount to the position command of the motor at the time of reversal. Japanese Patent No. 3389417 discloses to find in advance the relationship of a feed speed before reversal and a correction amount for an amount of movement until reversal at the different feed axis and to use this relationship as the basis to find the correction amount. Further, Japanese Patent No. 3703664 discloses to update the correction amount in accordance with the time elapsed after reversal.
In this regard, FIG. 11 is another view for explaining backlash. FIG. 11 shows an initial position where neither of the projecting parts A1 and A2 of the moving part WA are engaged with ends of the projecting part B of the driven part WB. If the moving part WA is made to move in the left direction from such an initial position, the moving part WA moves by exactly a distance C1 shorter than the backlash C whereby it engages with the other end of the projecting part B of the driven part WB (see FIG. 10C). Such a phenomenon easily occurs when the backlash is relatively large.
However, if adding a correction amount corresponding to the backlash C to the position command of the motor even in such a case, the correction amount would become excessively large. To keep the correction amount from becoming excessively large, it is also possible to prepare a predetermined correction amount which is smaller than the backlash C. However, if making the correction amount too small, sometimes the correction amount becomes insufficient at the time of reversal.
The present invention was made in consideration of such a situation and has as its object to provide a motor control system which can prepare the optimum correction amount even if the amount of movement from the initial position is smaller than the backlash.